Oil sand, such as is mined in the Fort McMurray region of Alberta, generally comprises water-wet sand grains held together by a matrix of viscous bitumen. It lends itself to liberation of the sand grains from the bitumen, preferably by slurrying the oil sand in heated process water, allowing the bitumen to move to the aqueous phase.
For many years, the bitumen in the McMurray sand has been commercially removed from oil sand using what is commonly referred to in the industry as the “hot water process”. The oil sand is strip-mined and conveyed on belt conveyors, often several kilometres in length, to an extraction plant. At the extraction plant, the oil sand is mixed with hot water (95° C.) and a small amount of caustic in a rotating horizontal drum or tumbler, where oil sand conditioning occurs. Here, the larger lumps of oil sand are ablated or digested and the released bitumen flecks coalesce and attach to air bubbles (referred to as “conditioning”). On leaving the tumbler, the conditioned slurry is diluted with additional hot water and retained under quiescent conditions for a prolonged period in a primary separation vessel (“PSV”), where the bitumen forms a froth that rises to the top of the vessel.
However, use of belt conveyors extending from the mine site to the extraction plant produced a number of problems. First, belt conveyors are expensive to install, operate and maintain. Further, as the mining area increases in the Fort McMurray region, the location of mining faces became more and more remote from the extraction plant, requiring more and longer belt conveyors to transport the mined oil sand.
The introduction of a pipeline to convey an aqueous slurry of the oil sands from the mine site to the extraction plant was a major advancement in the art. Surprisingly, it was found that much of the oil sand slurry conditioning takes place during transport of the slurry through the pipeline. Hence, the pipelined slurry could be fed directly to the PSV, thereby eliminating the need for large tumblers at the extraction plant. Nevertheless, the oil sand must still be satisfactorily blended with heated water at the mine site to produce a slurry capable of being conveyed through a pipeline (referred to as “pumpable slurry”) for transport and conditioning therein.
One slurry preparation system for producing pumpable slurry is referred to as the mixer circuit and is taught in Canadian Patent No. 2,000,984 and U.S. Pat. No. 5,264,118. The stationary mixer circuit comprises a vertically oriented mixer vessel forming a cylindrical, open-topped mixing chamber. A vortex is formed in the mixing chamber by tangentially feeding recycled slurry and to this rotating vortex is added oil sand and fresh water. However, the residence time in the mixer circuit is short (e.g., less than 30 seconds), resulting in a higher than desirable number of larger oil sand lumps, which are incapable of being pumped through the pipeline, and as such have to be removed. Further, the mixer circuit is very large and not amenable to being readily moved.
There is a need for an efficient oil sand slurry preparation system comprising a slurry preparation means for suitably digesting oil sand lumps to produce a pumpable oil sand slurry and a means for recycling oversize rejects, which rejects include large oil sand lumps, back to the slurry preparation means for further digestion, thereby reducing the overall amount of oversize rejects remaining. Preferably, the system is relocatable and can be periodically moved from location to location as the mine face advances.
Thus, the present invention is directed towards a relocatable oil sand slurry preparation system, which satisfactorily blends the oil sand with heated water to yield a consistent, dense (e.g., 1.5-1.65 g/cc), aerated oil sand slurry that is amenable to pipeline conveyance while substantially reducing the overall amount of oversize rejects.